The field of the invention generally relates to furnaces having condensing or recuperative heat exchangers, and more particularly relates to heating systems that include such furnaces and also include space air heat exchangers and/or domestic hot water storage tanks.
Nonrecuperative furnaces transfer only sensible heat from the combustion products. That is, the combustion products are not cooled below their dew point before being exhausted. As a result, condensation does not take place within a nonrecuperative heat exchanger, and the process is commonly referred to as a dry process.
In contrast, recuperative furnaces not only transfer sensible heat from the combustion products, but also transfer heat of condensation as the combustion products are cooled below their dew point so that condensation takes place. As is well known, an advantage of a recuperative heat exchanger is that it generally operates at a higher efficiency because a larger percentage of the heat is extracted from the combustion products. For example, a recuperative heat exchanger may increase overall furnace efficiency to approximately 95% whereas nonrecuperative furnaces typically operate below 90% efficiency. Another advantage of a recuperative furnace is that it enables the use of inexpensive exhaust venting such as, for example, PVC pipe rather than conventional chimneys.
As is known, recuperative heat exchangers are subject to corrosive attack by acidic condensate formed therein. In combusting natural gas, and even to a greater extent fuel oil, a number of potential acid forming gases are produced. Although these gases are typically noncondensible at the operating temperatures of the recuperative heat exchanger, they are absorbed by water vapor condensate thereby forming acids. For example, carbon dioxide forms carbonic acid, nitrogen dioxide forms nitric acid, hydrogen chloride forms hydrochloric acid, and hydrogen fluoride forms hydrofluoric acid. In addition, sulfur dioxide will condense within a recuperative heat exchanger thereby forming sulfurous acid. The acidity of the condensate is further increased when water condensate evaporates leaving behind concentrated acids which corrosively attack the heat exchanger. Such concentrations typically occur at transition regions between dry and wet surfaces within the heat exchanger.
In one prior art recuperative furnace, the burner module includes a plurality of tubes that are disposed radially around the burner element. The combustion products are directed radially past the tubes and heat is transferred to a glycol mixture which is circulated through the tubes. The glycol mixture flows through a loop that includes a remote fin and tube heat exchanger that operates in a noncondensing mode. In order to extract further heat from the combustion products to provide higher efficiency, the combustion products are exhausted through a stainless steel recuperative heat exchanger that is positioned below the remote glycol heat exchanger. Space air is directed up through the recuperative heat exchanger first for preheating before being directed through the glycol heat exchanger. In addition to fabricating the recuperative heat exchanger from stainless steel to reduce corrosive attack, a flushing system directs water through the combustion products path after each burner cycle. In the event that domestic hot water is to be heated by such system, the glycol mixture is diverted through a tube coil which passes through a hot water tank. A drawback of the above-described system, however, is that it is relatively expensive to fabricate. Also, the hot water tank may be subject to deposits on the outside of the glycol conveying tube coil which interferes with the transfer of heat from the glycol mixture to the domestic water.
My U.S. Pat. No. 4,726,353 describes a recuperative furnace wherein the dew point of the combustion products is elevated before introducing them into a heat exchanger. In such manner, condensation within the heat exchanger is greatly increased, and the internal surfaces are maintained in a substantially continuous wet mode of operation. With such arrangement, the transition regions between hot and cold spots are greatly reduced or eliminated, and the temperature to which the heat exchanger is exposed is reduced. In one embodiment, a fin and tube heat exchanger is provided and the combustion products are directed across the fins while domestic hot water is channeled through the tubes. The condensate forming on the fins drains downwardly into the reservoir where it reevaporates.